This invention relates to high conductivity semiconductor films and more particularly to II-VI reactively sputtered films using Zn as at least one of the group II elements and any one or more elements selected from group VI.
There has been some research on sputter deposition of semiconductor materials, such as CdS, CuInSe.sub.2, Cu.sub.2 S, using magnetron sputtering techniques. Sputtering offers great promise for films deposited over large areas from which heterojunction solar cells of high efficiency may be fabricated. However, a significant problem experienced is sufficient reduction of the resistivity of the sputtered film for use in device application. Unless sufficiently low resistivity films can be produced, the material is not suitable for solar cells, for example, because the I.sup.2 R losses in the material limit the solar cell panel efficiency.
Zinc selenide (ZnSe) would be an interesting semiconductor for heterojunction solar cells, and light emitting diodes because of its high bandgap (2.67 eV), lattice constant (5.6687.ANG.), thermal expansion coefficient (7.times.10.sup.-6 .degree.C..sup.-1), and electron affinity (4.09 eV). However, when deposited as a thin film by a variety of techniques, ZnSe is usually too resistive for practical use in a solar cell without the impractical step of annealing in excess Zn vapor for many hours, or even days. For example, the application of ZnSe films to photovoltaics recently has included Zn.sub.3 P.sub.2 thin films as the photon absorber, using the vacuum evaporation of ZnSe, treated to have excess Zn, and a dopant such as Ga or In. Deposited ZnSe films not treated to have excess zinc have lateral resistivities of 10.sup.8 ohm-cm or more. Films doped with Ga, but not In, could have this resistivity lowered to slightly less than 1 ohm-cm only by annealing in excess zinc using a Zn vapor at 500.degree. C. for 24 hours.
Complimentary research has used close-spaced vapor transport in the presence of hydrogen for ZnSe deposition. The source material is ZnSe powder, sometimes including pure Al, and a separate Zn source placed in the reaction tube. However, deposited films not further treated had resistivities above 10.sup.5 ohm-cm, and annealing the films of ZnSe:Al in Zn vapor only brought the resistivity down to 800 ohm-cm at its lowest. Co-evaporation of Zn and Se with dopants has also been attempted without better results.
It is necessary to obtain low-resistivity films for heterojunction device application in order to develop a higher built-in potential at the junction, and to minimize series resistance and poor contact effects. For photovoltaic devices, the ZnSe resistivity should be 10 ohm-cm or even less (n.apprxeq.10.sup.16 cm.sup.-3).
The primary reason for the difficulty of depositing low resistivity ZnY films, where Y is a selected group VI element such as S, Se or Te, is the strong tendency for autocompensation of shallow donor impurities by zinc vacancies. If the density of these excess vacancies can be minimized during growth, such as by maintaining near-perfect stoichiometry, or even Zn-rich conditions within the crystalline grains, controlled doping by Al, Ga, or In should be feasible.
Thin films of ZnSe have been deposited by: close-spaced vapor transport; organo-metallic chemical vapor deposition (OM-CVD); liquid phase epitaxy (LPE); molecular beam epitaxy (MBE); vacuum evaporation of the compound (VEC); and rf sputtering of the compound. In most cases, deposited films not further treated have resistivities of 10.sup.4 -10.sup.8 ohm-cm, except where GaAs substrates are used at relatively high deposition temperatures. In those cases, out-diffusion of Ga from the GaAs substrates is probably the cause of resistivities as low as 0.05 ohm-cm.
It would be desirable for solar cell application to deposit doped ZnSe films on a substrate that will serve as the window to the solar energy, for example, glass. The resistivity of such a doped film can be lowered by subsequent annealing in excess Zn vapor. However, an annealing process is unacceptable for solar cell fabrication for reasons of cost, and also unacceptable in the case of ZnSe deposited on semiconductors such as GaAs,Ge, Zn.sub.3 P.sub.2 or CdTe for other device structures because of degradation of the heterojunction.
As is the case for many high bandgap II-VI compounds, it is suspected that the high resistivity of a zinc compound (typically 10.sup.6 -10.sup.8 ohm-cm) is caused by excess Zn vacancies compensating donor impurities. Consequently, an object of this invention is to deposit lower resistivity zinc semiconductor films by reactive dc magnetron sputtering of Zn in a gaseous mixture of a group VI hydride, such as H.sub.2 Se, and an inert element, such as Ar, using a suitable dopant, such as In.